The invention relates to a laser diode control method, preferably used in optical transmission systems, and to a control circuit.
Laser diodes are used in optical transmission systems as light emitters. For the current-light characteristic curve (FIG. 1) of a laser diode (henceforth only referred to as characteristic curve), three ranges can be distinguished:
A curve portion A having a slight slope, in which there is no emission yet of coherent light; the so-called threshold area B, which has a knee in the characteristic curve; and finally a steeper portion C of the characteristic curve having a constant slope, in which there is a linear correlation between the drive current and the optical power output.
In order to improve the operating condition it is possible to allow a bias current I1 to flow through the laser diode, as a result of which an operating point AP is defined on the characteristic curve K3. The operating point of the bias current will be selected such that it is situated in the threshold area of the characteristic curve or above that area so as to avoid delay effects. However too large a bias current will also be avoided in view of the shortening of the useful life of the laser diode and the loss of sensitivity of the receiver due to additional current noise. However, with a constant bias current this operating point on the characteristic curve will change as a result of thermal drift and the drift caused by the aging process.
From European patent application 0 061 034 a circuit is known, in which the optical light output of a laser diode is measured by means of a photodiode and is controlled by digital circuit elements to presettable values, by increasing the bias current in steps until the presettable optical minimum output is supplied by the laser diode. In this fashion a presettable operating point is determined. If the laser diode is modulated by adding modulation current and when doing so the maximum output is exceeded, the bias current is reduced in a stepwise fashion, until the average light output level has dropped below this maximum level again. If the light output level drops below the minimum output level, the bias current is again increased accordingly.
A condition for a proper controllabity, as stated in this Patent, is the fact that the average optical modulation power caused by the modulation current is slightly smaller than the difference in output level of the set maximum and minimum values. The average light output level can only be adjusted indirectly with the aid of the minimum and maximum levels. As a readjustment can only be effected when the top level is exceeded or the bottom level is fallen short of, respectively, the average light output level is not controlled at all as long as it is situated between these two levels.
As laser diodes shown large mutually variations, the operating point has to be determined separately for each individual diode. Thereto, from the characteristic curve to be made for each individual diode, the operating point is selected in a manner according to the above points of view, the associated light output level is taken from the curve and given accordingly as a parameter in the digital circuit. The amplification of the modulation step can then still be adapted to the slope of the characteristic curve, so that the requirements on the modulation output are fulfilled. After each exchange of the laser diode this costly procedure has to be repeated. Also as a result of the drift on account of aging, however, such an adjustment entailing not insignificant costs is required.